×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Effective Area Method for rings + FEA question

Effective Area Method for rings + FEA question

Effective Area Method for rings + FEA question

(OP)
I have been looking into the Effective Area Method outlined in NAVFAC DM7-02 for rectangular and circular footings. However, for instances of designing foundations for silos or similar structures, ring type foundations are typically used (circular foundation with an outside and inside radius). Does anyone have any experience with or documentation on how to utilize the Effective Area Method for shapes like that? I know it CAN be done as I've seen results for it, but I've never been able to determine the method applied.

Also, for use of FEA for large foundations, I've found a few issues with designs. Typically, I use a simple P/A + My/I for determining service bearing. When using FEA, the same foundations that work under my method tend to have local bearing failure due to concentrated pressure directing under larger loads. Is this typical? Are there provisions to deal with this out there?

RE: Effective Area Method for rings + FEA question

I used to write geotech reports for grain elevators. The elevators were typically built on continuous mat foundations 2 to 3 feet thick that extended about 2 feet beyond the walls of the slip-formed grain cells. Large settlements were common, but the structures were very rigid and tolerant of settlement so long as they didn't tilt too much. For an especially compressible site, I suggested overexcavating and replacing about 10 feet of the existing soil. The designer said he would rather make the mat wider and extended it about 8 feet beyond the walls. The extension broke off along one side. I believe this occurred because the contact pressure under a rigid foundation is higher near the edges than under the middle. Elastic analysis shows this; see Figure 7.3 in "Elastic Solutions for Soil and Rock Mechanics" by Poulos and Davis.(It's available on line for free) The designed had assumed a uniform bearing pressure.

Perhaps your FEA model is showing something that really happens. There is local shear failure of the soil under the edges of the foundation.

RE: Effective Area Method for rings + FEA question

As a very crude (and possibly conservative) approach, I'd calculate the effective area as if it were a solid foundation and then deduct the area of the hole. If you were to take this approach then you'd need to be careful to ensure the deducted hole area is only that area which would have been in contact with the soil if it was solid (since the hole area not in contact with the soil would've already been deducted when considering the effective area of a solid foundation).

Below is a picture to explain what I mean (based on Sargazi and Hosseininia - Bearing capacity of ring footings on cohesionless soil under eccentric load):



Of course, the above approach implicitly assumes the loading on your ring foundation is comparable to the loading on a solid foundation (i.e. typically in the middle of the foundation). This may not be the case for your ring foundation, so more detailed analysis may be warranted.

On your second point about the FEA: could you share an image to explain what you mean? The response by aeoliantexan seems reasonable - I just want to see what you are describing.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close